Internal combustion engines are used extensively for a variety of purposes. The transportation infrastructure relies almost exclusively on the use of engines to provide power for mobility. Electrical power generation also relies heavily on internal combustion engines.
The prolific use of engines in our society has created a number of issues, one of which is the ever-increasing amounts of combustion by-products being emitted. Although today's engines operate with much lower emission levels than previous generations of engines, the rapidly increasing numbers of engines being used creates the need to reduce emission levels even more.
Governments around the world recognize this problem and are taking regulatory steps to address the emission levels of engines. For example, levels of oxides of nitrogen (NOx), hydrocarbons (HC), carbon monoxide (CO), and smoke, among others, must be reduced drastically to meet evolving government standards.
Spark ignition engines, by the nature of their operation and the types of fuel used, tend to produce low levels of NOx and particulate emissions. Compression ignition engines, for example diesel engines, generally produce higher levels of NOx and particulate emissions. Diesel engines, however, are still popular in use because they provide higher thermal efficiency than their spark-ignition counterparts, and thus offer higher power output for work applications.
One attempt to reduce the emissions of compression ignition engines has been the use of aftertreatment systems to alter or remove the unwanted emissions from the exhaust of the engines. One form of aftertreatment technology that has shown promise in reducing the NOx emissions of compression ignition engines is NOx adsorber technology, a catalyst technology. However, the successful implementation of NOx adsorber technology has proven difficult. First, for sufficient NOx reduction at low temperatures, NOx adsorbers must have very high loadings of expensive noble metals. In fact, NOx adsorbers that operate successfully in low temperature conditions may require as much as twice the noble metal content of NOx adsorbers that only operate in higher temperature conditions. Second, the effectiveness of NOx adsorber technology in very low temperature conditions is questionable. To improve performance in these conditions, expensive and fuel intensive thermal management may be necessary. Third, the catalyst of a NOx adsorber is poisoned by sulfur, even at the current ultra low sulfur levels in fuel. This poisoning process reduces the overall lifespan of the catalyst.
Another attempt to reduce the emissions of compression ignition engines has been the use of homogeneous charge compression ignition (HCCI) combustion. Engines that operate in HCCI mode have generated much interest due to the potential to operate at high fuel efficiency while generating low combustion emissions. HCCI engines differ from conventional diesel compression ignition engines in that diesel engines ignite fuel that is rich, i.e., highly concentrated, in an area in a combustion chamber, while HCCI techniques create a dispersed homogeneous fuel/air mixture by the time of combustion. Combustion of a homogeneous fuel/air mixture allows an engine to operate such that emission by-products are significantly reduced. However, successful implementation of HCCI combustion at all engine load conditions has proven difficult. In high engine load conditions, HCCI combustion causes high mechanical loading of engine parts due to a higher peak cylinder pressure than traditional diesel combustion. Engine components having commonly used material compositions may not be able to withstand these higher pressures. Also, in order to control the timing of HCCI combustion in higher load conditions, significant structural changes may need to be made to the engine, including, for example, mechanisms for varying the compression ratio of the engine. One strategy for utilizing HCCI combustion is disclosed in U.S. Pat. No. 6,561,157, issued to zur Loye et al. on 13 May 2003. In the '157 patent, PCCl combustion, similar to HCCI combustion, is used during some portions of an engine's operation, and other combustion modes, such as standard diesel mode, spark ignited mode, and homogeneous charge duel fuel transition mode, are used during other portions of the engine's operation.
The present invention is directed to overcoming one or more of the problems as set forth above.